This invention relates to an ultramicroscopic spectrometer for determining the size, concentration and index of refraction of aerosol particles which are aerodynamically focused into an aerosol jet by an aerosol jet generating system. The spectrometer is arranged by means of a mount within a receptacle, the wall of which has a first passage for the incident illuminating beam and a second passage for the information beam of the aerosol jet. The axes of the illuminating beam, the information beam and the aerosol jet intersect in a measuring field.
The aerosol particles contained in the environmental atmosphere have particle diameters spanning a range of many orders of size. An important size range within atmospheric physics and industrial hygiene is constituted by submicroscopic aerosol particles.
The classical processes for the analysis of particle magnitudes comprise the separation of particles onto a proper specimen carrier and a subsequent evaluation of the specimen by a light microscope or an electron microscope. These processes are very time consuming due to a visual counting of the particles. Further, the light microscope must be generally considered unsuitable for the submicroscopic particle range, since the formation of sharp images of the particles below the wavelength of light is not possible.
For the above reasons the ultramicroscopic process was developed for particles whose image could not be sharply reproduced. The ultramicroscopic process determines the particle size not by measuring the dimension of the particle image, but by measuring the intensity of the scattered radiation. The use of light scattering for size determination is particularly advantageous in case of ultramicroscopic particles (whose diameter is smaller than the wavelength of the light used), because the intensity of the scattered light in this order of size increases in a direct proportion (that is, it increases monotonically) with the particle size. This unequivocal relationship between scattered light intensity and particle diameter exists for both monochromatic light and white light.
Further, particles which are formed of materials with different real components of the index of refraction differ in their scattered light intensity by a constant factor which is independent from the particle diameter and which may be correlated unequivocally with the index of refraction. The influence of the self-absorption of the particles (complex index of refraction) on the light scattering is, in this size range, substantially negligible. Particles which deviate from the spherical shape are, in the light scattering, associated with the diameter of a sphere which has the same volume as the non-spherical particles.
In a conventional arrangement, however, ultramicroscopy can utilize the properties of light scattering (which are advantageous for the size determination) only in a limited manner, since the background scattering of the specimen carrier significantly restricts the lower limit of detection sensitivity and further, the conventional examining arrangement does not allow a substantial automation of the evaluation.
On the other hand, for measuring the concentration and size of aerosol particles, apparatuses are known which operate on the basis of light scattering and are known as optical particle counters. In this connection reference is made to A. E. Martens and K. H. Fuss, in the Journal "Staub-Reinhaltung der Luft," Vol. 28, p. 229 (1978). According to the process described in that publication, the particles are passed individually and sequentially through an optical gate, the scattered light flash emitted by each individual particle is photoelectrically recorded and the particle size is determined by the magnitude of the electric pulse. The measure for the concentration is the number of counting pulses per unit time. The advantages of an optical particle counter reside in the fact that the particles may be analyzed as they are carried by air and thus a preceding separation onto a specimen carrier need not be performed and further, based on the photoelectric recording of the light flashes, a high-grade automation may be effected.
In their present state of development, however, the optical particle counters have a number of disadvantages. Thus, by using conventional incandescent lamps as light sources and because of imprecise optical beam guidances, the scattered light intensity is too weak or the background scattering is too high for individually sensing particles that have a size below the wavelength of light. The principal measuring range of these apparatuses is thus above the wavelength of light.
For particle sizes above the wavelength of light the relationship between the scattered light intensity and the particle diameter is not unequivocal when monochromatic light is used. Thus, white light and a special scattered light angle range has to be selected to obtain an unequivocal, monotonously increasing calibration curve in a predetermined range of particle size.
Further, in optical particle counters used heretofore little attention has been given to the preparation of the illuminating beam and the guidance of the aerosol through the measuring field. The result has been a small resolving power of these apparatuses. For particle sizes above the wavelength of light, the light scattering is not a volume phenomenon but a surface phenomenon. Thus, the influence of the particle shape on the results of measurement increases inasmuch as irregularly-shaped articles are no longer assigned the diameter of a sphere of the same volume.